In traditional SEM tools, beam current is determined by intercepting the beam at some plane either in the gun or column. This invasive beam monitoring requires the beam to be intercepted for its current to be determined, and thus no beam is striking at the image plane and the SEM column is not imaging. To get around this, a certain “duty cycle” is imposed on the system if the beam current is to be monitored periodically. This negatively impacts the throughput of the SEM system.
In current methods of obtaining beam position, the beam position with respect to the optical axis inside the column is determined by a “wobble” function that deliberately perturbs one or more elements in the column from their nominal value to determine the beam position. When set in the wobble function the column, cannot be employed for imaging. Hence, this method also imposes a duty cycle, where a fraction of the useful imaging time is lost to a specific monitoring or tuning function that has to be implemented periodically as the beam drifts.
Another important beam characteristic many SEM tools can collect is an energy spectrum of the primary electron beam. All present methods of energy spectrum collection known to the inventors of the present disclosure involve an invasive means of energy spectrum detection such as directing the beam into a detector array placed at a dispersive plane in the beam path. The electron beam is captured and ends at the detector plane of a spectrometer.
In particle accelerators you use a pair of plates that produce an image charge as a beam containing a bunch of charges passes through it. The image charges produce pulses as the beam passes through it. The difference between pulses from the two plates can determine the position of the beam relative to the axis between the plates. The sum of the pulses at the two plates is proportional to the bunch charge. This works well if the beam is pulsed and the peak current is quite larger, e.g., of order 1 amp to 1 kiloamp. For many charged particle beam tools, such as electron microscopes, the beam is continuous and the current is very weak (nanoamps).
For very charged particle beam short pulses (e.g., femtoseconds) electro-optic crystals are sometimes used to sample fields from bunched electron beams in particle accelerators. The change to the index of refraction of the electro-optic crystal can be probed with a laser beam.
It is within this context that aspects of the present disclosure arise.